The present invention relates to an optical receiving apparatus and, particularly, to an optical receiving apparatus to be used in a transmission of digital data in the form of burst in such as a passive double star optical communication system or an ESSANET communication system.
In a conventional optical receiving apparatus, an optical signal input to the receiving apparatus is converted into an electric current signal by a receiving element such as avalanche photo-diode and the current signal is amplified by a preamplifier. The preamplifier is constructed with feedback resistors and a peak hold circuit. A comparator compares the amplified current signal level with a predetermined threshold level and when it is higher than the threshold level, the signal is discriminated as a logical value "1" and, when it is lower than the threshold level, the signal is discriminated as a logical value "0". An example of such conventional optical receiving apparatus is disclosed in U.S. Pat. No. 5,025,456.
However, there are two problems inherent to the conventional optical receiving apparatus. The first problem is that duty factor of an output waveform is considerably varied due to analog error such as offset error of the preamplifier. The existence of offset error in a differential amplifier which is used in the preamplifier may be a source of level variation of the output waveform of the receiving signal. In order to remove the offset in the differential amplifier, it is possible to regulate the offset by initially regulating the threshold level, that is, reference voltage of the comparator. However, since it is difficult to regulate the offset by taking environmental condition of the apparatus such as ambient temperature and condition of a power source thereof into consideration, the offset may be changed externally necessarily and duty factor of the output waveform of the receiving signal may be varied. With such variation of duty factor of the output waveform of the receiving signal, a margin for data discrimination becomes substantially small particularly when the receiving signal level is low. As a result, the minimum input level of the optical receiving apparatus itself is limited.
In the optical receiving apparatus which processes digital data in the form of burst, it is general to extract a timing signal or clock signal on the basis of information of both a rising edge and a falling edge of the data since a high speed synchronization is necessary. Therefore, jitter of the extracted clock is increased and the synchronizing characteristics is degraded, due to the duty factor variation.
The second problem is that, since the differential amplifier includes a feedback loop, the design of the apparatus requires an enough margin for oscillation and, therefore, the signal frequency band and amplification of the differential amplifier are severely restricted.